This invention pertains to fuel cells for producing electricity for vehicle propulsion and stationary uses. More particularly, it pertains to integrated fuel cell processes using thermoelectric reforming, hydrogen gas (H2) purification, water-gas reactions, fuel cell stacks, waste heat management, and recycling of water and H2.
Fuel cells convert chemical energy contained in a fuel directly into electrical energy. Because the conversion does not involve conversion of heat into mechanical energy, fuel cell efficiencies can exceed the Carnot Cycle limit by at least a factor of two. Also because they do not involve air combustion, use of fuel cells can reduce local air pollution, reduce quantities of greenhouse gases in the atmosphere, reduce oil imports, and reduce noise. For example as discussed by Berlowitz and Darnell for a current mid-sized automobile about 18% of the energy in the fuel is converted to work to drive the wheels, whereas, a vehicle with a fuel cell, utilizes 36% of the fuel""s energy to achieve the same result. All fuel cells contain an anode and a cathode that are separated by an electrolyte. A hydrogen-rich gas is fed to the anode and oxygen is fed to the cathode. A catalyst separates the proton and electron in hydrogen atoms, allowing the protons to pass through a selective membrane. The electrons flow through an external circuit and combine with the oxygen ions and hydrogen ions, to form only water and electricity. The effectiveness of the chemical-to-electrical energy conversion is heavily dependent upon the choice of the electrolyte. Consequently, the electrolyte determines the category of the fuel cell.
Unlike batteries, fuel cells will continue to produce electricity as long as sources of hydrogen and oxygen are available. There are several options for supplying hydrogen to the fuel cell. Pure hydrogen can be stored locally and supplied to the fuel cell as needed. However, it is very expensive to transport and store hydrogen due to its low energy density. Another source of hydrogen is liquid or gaseous hydrocarbon fuels such as natural gas, methanol, ethanol, gasoline, diesel fuel, light hydrocarbons, vegetable oils, or biomass derived alcohols. To use a hydrocarbon fuel efficiently, it must be reformed into hydrogen and ultimately, carbon dioxide. The most common reforming methods include steam reforming, partial oxidation, and autothermal reforming.
Steam reforming uses steam and the hydrocarbon fuel to create hydrogen and carbon monoxide, which is further oxidized in a shift reaction to carbon dioxide. It uses an endothermic reaction, however, which reduces efficiency and limits the responsiveness of the system. Partial oxidation, such as taught by Werth in U.S. Pat. No. 5,925,322, utilizes air as the oxidant. However, air contains relatively large amounts of nitrogen that dilute the concentration of the hydrogen stream. Both steam reforming and partial oxidation typically employ sensitive catalysts to reduce temperature requirements and increase reaction rates. Contamination of the catalysts can limit their life, effectiveness, or both. Autothermal reformers are a combination of steam reforming and partial oxidation, and thereby have advantages of both as well as the shortcomings of both. Each of the above reformer tend to be large, heavy, and lack quick response to transient loadings, which is particularly limiting in on-board transportation applications.
Another, less common method of reforming hydrocarbon fuels is through the use of a thermoelectric device such as a thermal plasma, a microwave plasma, a plasma torch, or a flameless thermal pyrolysis systems. The benefits of a thermoelectric reforming method for the system are (1) the absence of air with the effect of dilution of hydrogen with nitrogen, and (2) rapid response times. The use of a thermoelectric reformer is also beneficial for the environment because it does not use combustion and consequently combustion gases are not exhausted. The use of a plasma reformer to produce hydrogen rich gas is taught is discussed by Cohn, et al. in U.S. Pat. No. 5,887,554 and by Bromberg et al. in U.S. Pat. No. 5,409,784. The use of a plasma reactor with microwave energies for the production of hydrogen from dissociation of hydrogen sulfide is taught by Harkness et al in U.S. Pat. No. 5,211,923. The use of a flameless thermal pyrolysis reactor to dissociate hazardous waste and hydrogen sulfide is taught by Wang in U.S. Pat. No. 5,614,156 and U.S. Pat. No.5,843,395, respectively. Thermoelectric devices, have shorter response times owing to the use of ions and electromagnetic fields to promote the dissociation of the process gas. In such devices, the ionization energy created in the reactor is much higher than the fuel energy; therefore, transient fuel loading changes will not be sensitive to the overall energy retained in the reactor. That is to say, reforming or conversion from fuel to hydrogen is very fast compared to a steam reformer or partial oxidation reformer. Thermoelectric reformers take advantage of fast ion-molecule reactions during the exothermic heating and involve ions having energies higher than the thermal energy content of hydrocarbon fuels. The independence of fuel loading for the thermoelectric device is particularly important for the on-board fuel-cell powered vehicle applications. Since the thermal plasma (or ionization) reaction is fast, a compact and light weight design can be achieved without compromising the power output. There are two ways to reduce the warm-up time of the reformer (1) increasing the power density, and (2) decreasing the thermal capacity of the reformer. The longer the warm-up time, the larger the battery or ultra capacitor and the more metal hydride that is needed to initially start up the fuel cell system (FCS). Note that the larger battery sizes and more metal hydride will increase the total weight of the FCS and the space needed.
In addition to the above characteristics, thermoelectric devices can reform a wide variety of hydrocarbon fuels. They are not xe2x80x9cpoisonedxe2x80x9d by fuel gas streams as are catalysts used in steam reformers, partial oxidation reformers, or autothermal reformers. Also, thermoelectric reformers are not sensitive to temperature as are catalysts used in the other types of reformers. The heat required to vaporize the methanol is nearly four times that of gasoline. Therefore, the current invention locates a fast mixing evaporator (mixer) in a high temperature zone using either waste heat from a carbon dioxide stream from a hydrogen separator or the fuel cell""s waste heat stream to maximize utilization of waste energy.
A rule of thumb for the energy balance of internal combustion engines (ICEs) is that 33% of the energy produces useful power, 33% is rejected from the coolant, and 33% is exhausted from the exhaust system. On the other hand, for a fuel cell system 40% of the energy produces usefil power, 50% is rejected from the coolant, and 10% is exhausted from the exhaust system. The coolant temperature of a fuel cell system is significantly lower, typically 80xc2x0 C., compared to an ICEs 120xc2x0 C. In general, a significant fraction of the waste heat of an FCS must be rejected at a temperature lower than the fuel cell stack temperature for condensate recovery. This creates a significant challenge for designers of fuel cell systems. In the instant invention this problem is addressed by incorporating a thermal management system to recover waste heat produced in the fuel cell system. This system will recover waste heat for heating a metal hydride hydrogen storage system, producing steam for vaporizing the liquid fuel, and providing an air heating source for the occupant compartment in a vehicle or in a building.
Presence of carbon dioxide in the fuel cell dilutes H2 and decreases the fuel cell""s efficiency. Also, the concentration of carbon monoxide in the stream entering the fuel cell should be in the range of 50-100 ppm for maximum efficiency in view of the state of the art fuel cell technology. The present invention has means for removing carbon dioxide and carbon monoxide before entry to the fuel cell.
The invention is a fuel cell process for producing electricity for stationary purposes or for electric-powered vehicles. A fuel cell produces electricity from hydrogen gas. A hydrocarbon fuel is input to the system and mixed with steam recycled from the fuel cell and a hot stream from the a hydrogen separator containing carbon dioxide, carbon monoxide and trace hydrocarbons. The carbon dioxide is then separated from the stream which is then preheated in a regenerative heat exchanger before entering a thermoelectric reformer where the hydrocarbon fuel and H2O react under high temperature to form hydrogen gas (H2), carbon monoxide, and carbon dioxide. The reformate stream is then mixed with water and directed to a high temperature water-gas shift reactor, then to the regenerative heat exchanger where heat is transferred to preheat the fuel-steam mixture entering the thermoelectric reformer. After leaving the regenerative heat exchanger, the reformate stream is directed to a low-temperature water-gas shift reactor and to the hydrogen separator. The water-gas shift reactors convert carbon monoxide to carbon dioxide and creates additional hydrogen gas. The hydrogen-rich stream leaving the last water-gas shift reactor is directed to the hydrogen separator where impurities are removed and is then directed to a condenser where excess water is condensed. The saturated hydrogen stream from the condenser then enters the fuel cell. Most of the electricity produced in the fuel cell is used for the electrical load. However, electricity produced in the fuel cell is also used to power the thermoelectric reformer and is stored in an energy storage device for subsequent use in starting up the system when used intermittently in a vehicle and for responding to transient demand peaks. Hydrogen entering the fuel cell that is not consumed there, is stored, most likely in a metal hydride storage system, for subsequent mixing with the hydrogen stream from the hydrogen separator and is then recycled in the fuel cell. H2O from the fuel cell is recycled for use in the water-gas shift reactors and is heated with waste heat from the fuel cell prior to being mixed with input fuel. The waste heat that is contained in the exhaust air stream is used to heat recycled water, input fuel, and the hydrogen storage device within the FCS, and to provide heat energy sources external to the FCS. Because the H2 reformer operates at high temperatures and more waste heat is produced than can be used internally, the extra heat can be recovered and used extemally. In a vehicle application, the extra heat can be used to provide space heating for the passenger compartment; in stationary applications, the this heat can be used to space heating and hot water heating in buildings. Also the water produced by the FCS is more than enough for internal use in the system. The extra water can be used externally. An integrated control system responds to the load and controls the rate at which fuel enters the system and flow rates in the system. This system also monitors the system for proper operation.
The objectives of the present invention is to provide a fuel cell system with a thermoelectric reformer with: (1) high conversion efficiency in the reformer; (2) a reformer that does not use air combustion; (3) multi-fuel capabilities and fuel flexibility; (4) fast response to initial startup and transient load demands; (5) high efficiency in the fuel cell in producing electricity; (6) waste heat management; (7) H2O and unused H2 recovery and direct use; (8) system automation; and (10) compact design for transportation applications.